Sure it does; it's completely clear. If you have a scene which averages mid-grey and has a histogram in the middle of the range when correctly exposed, and then you ETTR, you are using a longer exposure to get more light hitting the sensor (as much as you can without clipping highlights) and reduce noise. Which has exactly the same effect as reducing the ISO.

Quite so.

Sure. That's not exactly what Baz said though.

That's how it came across to me. And we all seem to agree that it makes sense.

OK - then perhaps it was my misunderstanding. Thanks for your patience. I'd have appreciated the same from Baz - but obviously he doesn't have time to explain his assertions by himself or he feels that some here are not worth it - which is a pity from someone partaking in a beginners forum.

In another discussion of this type recently about high-ISO shooting (link below), we were discussing pix taken at ISO6400 in which I had ETTR'd to help keep noise down as far as possible. It was pointed out that it would actually be better to use a lower ISO (say, 800) but underexpose by three stops - effectively, exposing to the left - and then lifting exposure in PP. This would give the same overall result in terms of IQ and noise - but would prevent highlight clipping. It's exactly the converse of ETTR.

And, as I've said, that depends a lot on what camera you use. You would definitely not be better off doing that with a Canon camera, for instance.

Could you explain why? As far as I can see if you underexpose by three stops (by using ISO 800 instead of 6400, but with the same exposure settings) you are just telling the camera not to multiply the signal by a factor of 8. Instead you do it yourself afterwards in PP (whilst protecting highlights). Why would this not work so well with a Canon?

Because Canon sensors have noisy readout when compared to the EXMOR based systems (which I'm guessing you were discussing). So, if you underexpose by three stops and then bring it back up in post, you will end up with a noisier image than you will if you simply use the 'metered' ISO.

I should add - this is the case currently. That's not to say that sometime in the future things wont change. At the moment though, readout noise goes up noticeably as ISO drops with Canon's sensors, so just sticking to the 'metered ISO' usually gives the best results.

In practice, this has little consequence, and particularly so for this discussion, which is about ETTR (where you will be slightly 'overexposing' according to the metered solution - not going a number of stops under).